1. Field of the Invention
The present invention relates generally to printer apparatus and more particularly to gray scale recording type printer apparatus having a head drive system that can perform gray scale recording at high speed.
2. Description of the Prior Art
Either density modulation or area modulation is usually employed in conventional gray scale recording. In density modulation, each picture element or pixel is constituted by a single printing dot, and gray levels of an image to be printed are determined by adjusting the tone of each pixel. Density modulation can provide high resolution of the printed image. On the other hand, when a relatively great number of tones are to be printed, degradation of image quality tends to occur because of an unevenness or abrupt change of tone which is due to unevenness or roughness of the printing paper. Such tendency is increased if a substantially smooth printing paper is required, as in a thermal printer system.
On the other hand, in the case of area modulation, each pixel is constituted by a plurality of printing dots, and the tone is determined by the number of printing dots. Area modulation typically does not cause unevenness or abbrupt change of tone such as that occurring in density modulation. However, since in area modulation each pixel is constituted by a plurality of printing dots, the pixels tend to be too large to provide high resolution.
In order to provide better print quality, there has been proposed a method combining density modulation and area modulation in gray scale recording. In the proposed system, a plurality of printing dots are employed to constitute each pixel (as in area modulation), and the tone of each printing dot is modulated (as in density modulation). In this combined process a printed image of better quality can be obtained in comparison with a method employing density modulation alone or area modulation alone. However, in the combined method, extremely complex signal processing for each pixel of the original image is required, and the image data processing speed is severely limited by the complexity of the signal processing.